August 2014 LIP of the Month

Accreted Oceanic Plateaus in Japan

Yuji Ichiyama (Kochi Institute for Core sample Research, JAMSTEC)
e-mail: ichiyamay@jamstec.go.jp

Akira Ishiwatari (Center for Northeast Asian Studies, Tohoku University)
e-mail: geoishw@cneas.tohoku.ac.jp

Introduction

Oceanic plateaus and continental flood basalt traps comprise large igneous provinces (LIPs) of the Earth. They are the products of intensive volcanic activities caused by large-scale, very hot mantle plumes (e.g., Richards et al., 1989). The largest oceanic plateaus such as the Shatsky Rise, Ontong Java Plateau and Manihiki Plateau are located in the western Pacific Ocean, and are going to be subducted into trenches in the future. In fact, the Ontong Java Plateau is colliding with the Solomon Island Arc, and its thick basaltic lava piles crop out on land as a consequence of plateau accretion (Tejada, et al., 1996). Therefore, ancient oceanic plateaus on the already subducted plates are expected to be preserved only in orogenic belts as accretionary complexes or ophiolites. A well-known example is the Cretaceous Caribbean-Colombian Oceanic Plateau, where ultramafic volcanics such as komatiites and picrites are widespread (e.g., Kerr et al., 1997), and have provided an important constraint on petrological model for the genesis of large mantle plume in the Phanerozoic (Herzberg and O’Hara, 2002; Herzberg et al., 2007).

The geology of the Japanese Islands basically consists of nappe piles of Paleozoic to Cenozoic accretionary complexes and ophiolites (Isozaki et al., 1990; Ishiwatari, 1991). Recent geological and petrological studies of the Permian volcanics in the Mino-Tamba belt and the Jurassic volcanics in the Sorachi-Yezo and Mikabu belts have provided lines of evidence that these are accreted fragments of oceanic plateaus (Ichiyama et al., 2008, 2014). Dilek and Furnes (2011) proposed a new ophiolite classification, in which accreted plume-related complexes such as the Caribbean-Colombian Oceanic Plateau and the Mino-Tamba belt are newly classified as “P-type (plume-type)” ophiolites. Multiple-aged P-type ophiolites are widespread in the orogenic belts from the Far-East to Central Asia (Safonova et al., 2009; Erdenesaihan et al., 2013), and their investigations play an important role as geological records to reveal the timing of oceanic LIP formation in the Earth’s history and their deep igneous processes below oceanic LIPs. Here, we give a short review of the geological and petrological characteristics of the accreted oceanic plateaus in the Mino-Tamba, Sorachi-Yezo and Mikabu belts, Japan.

Early Permian Mino-Tamba Plateau

The Mino-Tamba belt occupies wide areas of the Inner zone of southwest Japan (Fig. 1). This belt consists of basal basaltic lavas with Permian to Triassic limestone and pelagic chert, and Jurassic terrigenous sandstone and mudstone. The basal tholeiitic basalts commonly occur as massive and pillow lavas (Fig. 2a), and possibly erupted in Early Permian (partly Late Carboniferous). Dykes and sills of highly enriched magnesian volcanics (meimechite-like picrite and ferropicrite) and related hyaloclastite layers are also present in Middle Permian chert (Ichiyama and Ishiwatari, 2005; Ichiyama et al., 2006) (Fig. 2b). Olivine-spinifex basalt, which is exactly the same rock as occurring in the Early Proterozoic Pechenga LIP in the Kola Peninsula (Hanski, 1992), also occurs with ferropicrites (Ichiyama et al., 2007) (Fig. 2c). Geochemistry of the basal basalts is characterized by flat trace element pattern in common with oceanic plateau basalts (Fig. 3a). On the other hand, the enriched volcanics show steep trace element patterns enriched in incompatible elements, which are comparable to those of HIMU (high-µ) ocean island basalts from the St. Helena Island and French Polynesian Islands (Fig. 3b). Sr and Nd isotopic compositions of the enriched volcanics also exhibit the same values as the HIMU basalts, indicating involvement of recycled oceanic crust (eclogite or garnet-clinopyroxenite) in their source mantle (Ichiyama and Ishiwatari, 2005; Ichiyama et al., 2006). The characteristic occurrence of meimechite and ferropicrite in continental LIPs (Gibson et al., 2000; Desta et al., 2014) and the geochemical similarities between the basal basalts and oceanic plateau basalts indicate that the volcanic rocks in the Mino-Tamba belt are fragments of an accreted oceanic plateau. Long travel time of the plateau from Early Permian to Late Jurassic (~130 m.y.) indicates that the plateau formed in the middle of the ocean. The paleomagnetism of the basal basalts show low inclinations indicating the formation in equatorial areas (Hattori and Hirooka, 1979), and low paleolatitudes (between 10 °N and10 °S) are recorded in the overlying Triassic chert (Ando et al., 2001).

Late Jurassic Sorachi-Yezo and Mikabu Plateaus

The Sorachi-Yezo belt stretches for 400 km in a N-S direction in central Hokkaido, northern Japan (Fig. 1). This belt is composed of abundant volcanic rocks with Late Jurassic pelagic chert, and can be regarded as an ophiolite together with a serpentinite mélange in the tectonically underlying Kamuikotan metamorphic belt (Ishizuka, 1985). In the Sorachi-Yezo belt, large bodies of cumulate and gabbro are not developed. The volcanic rocks are basaltic to picritic massive and pillow lavas and hyaloclastites (Fig. 4a and b). The basaltic rocks in the Sorachi-Yezo belt are similar in geochemical signatures to oceanic plateau basalts (Nagahashi and Miyashita, 2002). The Mikabu belt extends for 1,000 km in an E-W direction in the Outer Zone of southwest Japan, accompanied with the Sanbagawa metamorphic belt (Fig. 1). The Mikabu belt represents an ophiolitic mélange composed of variable-sized blocks of volcanic rocks with Late Jurassic chert, gabbros and ultramafic cumulates with their clastic matrices (Iwasaki, 1979; Saito et al., 1979). Residual peridotites have never been reported from the Mikabu belt. Radiometric ages of amphiboles in the ultramafic cumulates crowd around 140-150 Ma (Ozawa et al., 1997). The volcanic rocks in the Mikabu belt also include basaltic to picritic massive and pillow lavas and hyaloclastites.

The picrites in the Sorachi-Yezo and Mikabu belts show olivine-phyric texture with a groundmass of quench-textured (microspinifex) olivine and clinopyroxene (Fig. 4c). The olivine phenocrysts in the picrites from the Sorachi-Yezo and Mikabu belts are characterized by high Fo (=100Mg/[Mg+Fe]) and NiO contents (up to Fo₉₄ and 0.45 wt.%, respectively), and show gently curved trends following the mantle array in the Fo-NiO diagram, as in the case for Gorgona picrites and Archean komatiites (Ichiyama et al., 2012, 2014) (Fig. 5).

The picrites are very magnesian and nickeliferous (up to 30 wt.% and 1,900 ppm, respectively), and are depleted in Al₂O₃ and incompatible elements. The MgO contents of the parent magmas estimated from the Sorachi-Yezo and Mikabu picrites are 21-27wt.%, and the source mantle potential temperatures (T_p) of 1650-1700 °C are calculated (Ichiyama et al., 2014). These unusually high T_p mean that the Sorachi-Yezo and Mikabu belts are fragments of accreted oceanic plateaus formed by unusually hot, large mantle plume. In particular, these Sorachi-Yezo and Mikabu Plateaus represent the highest T_p calculated among Phanerozoic LIPs (for instance, 1500-1620 °C in the Caribbean-Colombian Oceanic Plateau and ~1550 °C in the Ontong Java Plateau; Herzberg and Gazel, 2009), which are rather close to the estimation for Archean komatiites (1650-1800 °C; Herzberg et al., 2010).

  The radiometric and biostratigraphic ages of the Sorachi-Yezo and Mikabu Plateaus indicate their coeval formation in the Late Jurassic Pacific Ocean. Oolitic sediments observed in the Sorachi-Yezo and Mikabu belts (Iwasaki, 1979; Takashima et al., 2006) provide evidence for the formation of these plateaus in equatorial areas. The paleomagnetism of the basalts indicates the situation in low paleolatitudes (Hoshi and Takashima, 1999), and the paleolatitude of 16.7° is estimated from the forearc sediments of Cretaceous Yezo Group underlain by the chert-basalt sequence (Tamaki et al., 2008). The Sorachi-Yezo and Mikabu Plateaus were probably formed in oceanic areas adjacent to each other. Kimura et al. (1994) suggested that the Sorachi-Yezo Plateau and Shatsky Rise were a twin-pair formed at the triple junction of the Pacific-Izanagi-Farallon Plates in Late Jurassic. If this is the case, it is possible that these three plateaus were produced by the same plume activity. However, their short travel time from Late Jurassic to Early Cretaceous (~20 m.y.?) suggests that their formation sites were possibly proximal to the subduction zone. Further investigation of the Sorachi-Yezo and Mikabu Plateaus will contribute in understanding the genesis of the Shatsky Rise, tectonics in the Jurassic Pacific Ocean and characteristics of Phanerozoic mantle plumes.

Concluding remarks and future perspectives

Accreted oceanic LIPs of the two different ages (Permian and Jurassic) have been recognized as P-type ophiolitic complexes in Japan. These LIP fragments provide us an understanding of primary magma composition, mantle potential temperature and mantle source heterogeneity. The accreted oceanic plateaus in Japan were probably constructed in equatorial areas, analogous to the current large mantle swell in the south Pacific (Pacific Superplume of Maruyama et al. (1997)). Further investigation of P-type ophiolites in the circum-Pacific orogenic belt will reveal compositional, thermal, spatial and temporal variations of the Pacific Superplume activities.

  Moreover, study of the terrestrial LIP rocks is also important in planetary geology. Plume magmatism is known to be more dominant in the other Earth-like planets than in the Earth, and evidence of post-bombardment LIP volcanism was reported from Mercury (Head et al., 2011). The volcanic rocks covering Mars are dominated by ferropicrite, komatiite, and related basaltic rocks (Filiberto, 2008; Gellert et al., 2006; Fleischer et al., 2010; Zipfel et al., 2011), which may also prevail on Mercury. Oceanic LIPs of the Earth are closer to those extra-terrestrial LIPs in respect to the absence of the underlying granitic crust, and their accreted fragments provide us a chance to study deep part of the LIP edifices as well as long term history of LIP magmatism that are essential in the future planetary LIP studies.

References